Microelectronic elements such as semiconductor chips are frequently manufactured by placing semiconductor wafers within a vacuum chamber of a chemical vapor deposition (CVD) reactor and then depositing layers of material on the wafers. During this process, one or more wafers are placed within the vacuum chamber and reactant chemicals in gaseous form are caused to flow over the wafers in controlled quantities and at controlled rates so that an epitaxial layer is grown on the surface of the wafer.
The reactant chemicals, frequently called precursors, are typically delivered to the vacuum chamber by placing the reactant chemicals in a device known as a bubbler and then passing a carrier gas through the bubbler. The carrier gas picks up molecules of the reactant chemicals as the gas passes through the bubbler to provide a reactant gas. The bubbler may include adjustable controls for modifying the concentration of the reactant chemicals in the carrier gas. The reactant gas is then fed to the CVD reactor thorough a mass flow controller which, in turn, delivers the reactant gas through associated plumbing to a flow flange and into the vacuum chamber.
CVD reactors have various designs, including horizontal reactors in which the wafer is mounted at an angle to the impinging reactant gases; horizontal reactors with planetary rotation in which the reactant gases pass across the wafers; barrel reactors; and recently, vertical high-speed rotating disk reactors in which the reactant gas is injected downwardly onto a substrate surface which is rotating within a reactor. These types of CVD reactors have been used successfully to grow a wide array of epitaxial layers, including various combinations of semiconductor thin-film devices and multi-layered structures such as lasers and LED's.
Vertical CVD reactors in which the wafers are located on a rotating disk include those shown in Hitchman et al., "Chemical Vapor Deposition. Principles and Applications," Academic Press, 1993; and Tompa et al., "Design and Application of Large Area RDRs, " III-Vs Review, vol. 7, no. 3, 1994. In these reactors, various techniques have been disclosed for providing a uniform flow of reactant gases over wafers. These include the use of a fine wire mesh, such as that shown in Wang et al., U.S. Pat. No. 4,997,677. In the '677 patent, the reactant gases are mixed in an antechamber before being passed through a fine stainless steel wire mesh. As such, the reactant gases chemically interact in the antechamber before being introduced into the reactor chamber. However, in certain applications the chemical interaction of the reactant gases may be undesirable because the gases may prematurely react before they reach the wafer. In order to avoid these problems, antechambers have been divided into segments for each reactant gas, as disclosed in the above-identified Tompa reference and in European Patent No. 687,749. In the '749 patent, separate parallel chambers are provided in separate planes for carrying separate reactant gases which are fed through individual conduits and into the reactor chamber. Premature reaction may nevertheless occur in such systems after the reactant gases are introduced into the vacuum chamber and/or during flow of the gases to the wafer.
The conditions or parameters under which the reactant gases are introduced into the vacuum chamber have a dramatic affect upon the characteristics of the epitaxial layers grown on the wafers. These parameters include material viscosity, density, vapor pressure, the flow path of the reactant gases, chemical activity and/or temperature. During research and development efforts directed to growing new types of epitaxial layers, researchers frequently modify the above-identified parameters and study the effects of such modification, in order to optimize the quality of the epitaxial layers grown on the wafers. One parameter which is frequently altered during research and development efforts is the flow path of the reactant gases. For example, the flow path of the gases may be modified by changing the design of the flow flange used to introduce the gases into the chambers. The researchers then study the epitaxial layer grown on the wafer to determine the optimum flow path for growing a particular type of layer. During this trial and error process, the flow flange is continuously modified until the best flow path for a particular type of epitaxial layer is identified.
Current flow flange designs introduce the reactant gases into the vacuum chamber at a single height. However, when growing certain types of epitaxial layers (e.g., epitaxial layers including oxides) the reactant gases must be maintained separately from one another until they reach, or just prior to reaching, the surface of the wafer. Thus, the introduction of reactant gases at a single height above the wafer may result in a premature chemical reaction taking place away from the wafer. Such a reaction, if allowed to commence too far from the wafer, typically results in the production of an inferior epitaxial layer. On the other hand, if the reactant gases are separated from one another for too long a period of time, then the reactant gases may not have sufficient time to fully react with one another before reaching the wafer. This will also result in the growth of inferior epitaxial layers. Thus, there is a need for a CVD reactor having an adjustable flow flange so that during the development of new growth systems, researchers may easily modify the flow path used to introduce the reactant gases into the vacuum chamber.
It is another object of the invention to provide an apparatus for delivering reactant gases at different distances from the wafers.
It is yet another object of the invention to provide a CVD reactor which can control the ratio of reactant chemicals flowing to different areas of the growth chamber without the use of needle valves or mass flow controllers.
It is still another object of the invention to provide a CVD reactor which allows for the rapid change of flow patterns for each of the reactant gases.
It is still a further object of the invention to provide a CVD reactor that creates a curtain of inert gas around the inner walls of the vacuum chamber to shield the walls from the reactant gases and to urge the reactant gases to move downwardly toward the wafer.